One of the primary advantages of moving to a flexible circuit design from a rigid board is the ability to package the flex in three dimensions, bending or folding into imaginative configurations and saving precious space in the final package. While flexible materials are robust and can withstand many flex cycles, nearly everyone can share a war story about the flex that didn’t originally perform as expected with copper cracking after installation.
I [Tara] remember an example from my early days in flex fabrication. We had built a fairly simple, double-sided flex with FR-4 stiffeners on both ends. After installation, the customer contacted us because the copper was cracking while it was being bent. In that case—and most cases even today—fabricators often have only a 2D view of the design. After some investigation of how the flex was being used, we made several recommendations to improve the performance of the circuit. Materials were adjusted, traces were re-routed to keep all of the traces on one layer in the critical bend area, polyimide stiffeners were added to guidethe bend exactly where it needed to be in the unit. Rather than plating electrodeposited (ED) copper onto the more flexible rolled-annealed (RA) copper, we button-plated and only plated ED copper on the pads and plated throughholes. The end result? Success! No more cracking.
Stories like this are not uncommon. Fabricators have quite an arsenal of tips and tricks to help improve flex life and avoid damage to flex materials during installation and use, yet are often building the circuits without knowledge of how it is going to be used in the final application. While our intention is to share some of the common methods of improving flexibility, we also want to strongly encourage everyone to communicate the flex and bend areas in the fabrication drawings or have discussions with your fabricator prior to release to take advantage of the knowledge they have to draw from and improve the performance of your new design.
Co-written with Anaya Vardya, American Standard Circuits
To read the full version of this column which originally appeared in the January 2019 issue of FLEX007 Magazine, click here.